In many tropical
countries, pressure on available land is increasing daily due to
the high rate of population increase, bringing, as a consequence,
increasing demands for food and the production of export crops to
help the balance of payments. As a result, there is a competition
for land use between traditional cattle production methods based
on grazing, and arable agricultural land dedicated to the
production of food crops and export crops.

In the light of these
developments, it is becoming increasingly important in the
tropics to integrate livestock with agriculture. One of the
objectives of this paper is to show how animals, and specifically
ruminants, can live and produce on the by-products and wastes of
agricultural crops.

The advantages of this
approach can be appreciated from a consideration of the potential
benefits that can result from integration of livestock and crop
production. These are:

the recycling of
animal wastes (manure) to produce biogas by anaerobic
fermentation, and later the application of the residual,
partially digested slurry as organic fertilizer for crop
production;

the saving in
energy that will result from the recycling process, and
the reduction in environmental contamination and health
hazards;

the possibility of
being able to exploit fully the potential of the tropics
for converting solar energy into biomass by means of the
production of crops of high biological efficiency with
multiple uses.

The possible integration
of livestock and crop production encourages decisions to be made
and action initiated in the following areas. In the first place,
there is the opportunity for better planning in the selection of
crops for growing in the tropics, the objective being to select
those with double or multiple use characteristics; in other
words, crops that have a capacity to produce food for human
consumption and large quantities of by-products for animal
feeding. On the other hand, the decision to integrate livestock
and agriculture makes it imperative to expand, rapidly and
widely, research and development into systems of animal
production that will encourage the more efficient utilization of
these by-products and wastes from crop production and
agro-industries.

As an example of crop
planning, one can take the existing situation in Central America,
where there is a strong tradition of growing maize as a staple
for human consumption. However, on the basis of the philosophy of
integrating livestock with agriculture, maize is by no means the
most appropriate crop, either in its capacity to produce grain
and byproducts, or in its mode of growth in the tropics, since
maize is susceptible to pests and also encourages land erosion in
areas of heavy rainfall. Table 1 shows the considerable
advantages that would be obtained by growing bananas instead of
maize, with the objective of multiple use for animal and
livestock needs. It would appear that the banana plant is capable
of producing more energy for human consumption (as starch) and at
the same time leaves behind in the form of by-products (the
forage, trunk, and leaves) very large quantities of biomass that
would be capable of supporting animal stocking rates similar to,
or even in excess of, those achieved normally in specialized
cattle production systems based on the use of pastures.

TABLE 1. Production of
Primary Product (Starch) and By-products from Maize and Banana
Crops

It is becoming
commonplace to plead for the development of appropriate
technology when we talk of the problems of developing countries
and the transfer of technology from the advanced countries
However, there is a strong argument in favour of this philosophy
in the general field of the feeding of ruminant animals on
tropical feeds and wastes.

The justification for
this approach can be seen in the many results obtained in the
feeding of tropical crops and byproducts to ruminant animals,
where performance levels have usually fallen short of
expectations in comparison with nutritional results obtained in
temperate climates.

The majority of
by-products and agricultural wastes of tropical origin are
characterized by the following factors: a. The carbohydrate
component consists of highly soluble elements (sugars) that have
a high rate of degradability in the rumen. It also has elements
of very low solubility (cell wall components) with low rates of
degradation by rumen microorganisms. In particular, most tropical
byproducts and wastes do not contain starch, which is an
important component of most feeds fed to livestock in temperate
zone countries. b. In addition, most tropical by-products and
wastes contain low levels of both total nitrogen and true
protein. The final effect arising from the combination of these
two sets of characteristics is a low voluntary intake by the
animal and this, in turn, leads to a low rate of productivity.

The nature of the
carbohydrate fraction in tropical feeds, as described above, and
the low nitrogen levels that necessitate incorporation of quite
high levels of non-protein nitrogen, means that the digestion
process in the animal must be principally by the route of
fermentation by rumen micro-organisms. This is because the sugars
are immediately fermented when they enter the rumen because of
their solubility, and because the animal secretes no gastric
enzymes capable of hydrolyzing cellulose and associated cell wall
components. Obviously, non-protein nitrogen, too, can only be
converted into protein by fermentation processes.

One would imagine that
such a situation would correspond closely to the environmental
niche for which ruminants were evolved. In fact, it has been
found that when the nature of the ruminant's diet is such that it
is obliged to pass through the rumen fermentation process, and
the only "feeds" presented for digestion in the small
intestine are the bodies of rumen micro-organisms, the voluntary
intake of feed (by the animal) is abnormally low, and as a
consequence the level of animal performance rarely exceeds
maintenance.

When voluntary intake is
low, rumen dilution, or turnover rate, is also low. This, in
turn, reduces the rate and efficiency of microbial growth and,
therefore, of microbial protein synthesis. Thus, with most
tropical feeds and byproducts, we have a poor performance
syndrome caused by low voluntary intake and low efficiency of
rumen function.

Results of recent
research have shown that three factors play determinant roles in
such dietary situations. The most important factor is
"by-pass" protein. This is protein of dietary origin
that is not, or to only a small extent, degraded by the rumen
micro-organisms and arrives intact at the duodenum, where it is
digested by gastric enzymes to its component amino acids that are
then absorbed. The value of such by-pass protein lies less in its
role as a direct source of amino acids than in the effect that it
has on over all voluntary intake and rumen function. Thus, the
higher the amount of by-pass protein, the greater is voluntary
intake. Furthermore, both rumen turnover rate and total flow-out
of the rumen also increase; ipso facto, efficiency of rumen
microbial protein synthesis is increased.

The second factor is the
roughage characteristics of the diet. These presumably act
through direct effects on rumen motility and rumen contractions,
and/or by providing a superior "ecosystem" in the
rumen. Evidence for the former is in the increase in intake and
rate of animal growth when sugar-cane leaves, employed as the
sole roughage in a liquid molasses diet, were chopped into large
pieces instead of being ground finely (1). Substantiation for the
improved "ecosystem" in the rumen are the observations
that both intake and digestibility on a basal sugar-cane diet
were improved when a highly digestible, but low-protein, roughage
in the form of banana stalk was included in the diet (2).

The third factor is
"by-pass" energy. The evidence here is less conclusive;
however, there are sufficient observations to justify belief in
the basic effect. Starch as a component of the dietary
supplement, and glucose infused directly into the duodenum, have
been the principal mechanisms used to effect the by-pass. It has
been known for some time that some starch sources (e.g., maize)
are not rapidly degraded by rumen micro-organisms, and that a
considerable portion of the starch passes on to the duodenum (3).
These observations were made on temperate zone, cereal-based
diets and little practical importance was attached to the
findings.

The digestion process in
the ruminant, outlining the separate pathways for fermentable and
by-pass nutrients, is shown in Figure 1. The quantities of
by-pass nutrients required will depend upon the rate of
productivity for the particular trait under consideration,
whether this is growth of tissue, as in production of meat, or
the secretion of milk.

Figure 2 shows how these
requirements for by-pass nutrients are related to the particular
stage of the production cycle of the animal. The neo-natal calf
and the high-producing milk cow represent two stages in the
production cycle when there is highest demand for by-pass
nutrients. Modern systems of cattle production, as developed in
the temperate zone countries, emphasize these requirements by the
early weaning of the calf and by encouraging very high rates of
lactation yield, and have led my colleagues and me to propose
that in tropical countries, where by-pass nutrients are
expensive, it is desirable to modify the management system so
that both early weaning and high individual milk production are
discouraged in favour of more moderate, less specialized
approaches (e.g., 4, 5).

Research on sugar-cane
and its by-products has provided ample confirmation of the role
of by-pass nutrients in the feeding of ruminants in the tropics.

The importance of
by-pass protein was established in Cuba in the late 1960s, when
it was demonstrated that, on a basal diet of molasses consisting
principally of sucrose and reducing sugars with small amounts of
soluble non-protein nitrogen supplemented with urea and
low-quality forage, the rate of animal performance was a
curvi-linear function of the amount of true protein fed (Figure
3). Only proteins of low rumen degradability were effective
(e.g., fish meal, torula yeast, and soybean meal), indicating
that they were acting by by-passing the rumen fermentation (6).

Work in Mexico (7)
showed that similar relationships could be demonstrated with
diets based on sugar-cane (which is mainly sucrose and cell wall
carbohydrates), and could be ascribed to the by-pass effects of
the protein in the supplement (8).

On this diet, the role
of by-pass starch also became evident. The data in Table 2 (9)
show how feed utilization efficiency was improved by feeding
maize containing by-pass starch, but not by molasses when
iso-energetic amounts of the two supplements were fed with
derinded sugar-cane. It has been shown that when rice polishings
containing 15 per cent protein, 13 per cent lipids, and 40 per
cent starch were added to a sugar-cane diet, at least half of the
starch passed unchanged to the duodenum (10). Furthermore,
glucose entry rates on the same diet were directly related to the
amount of rice polishings given (11).

TABLE 2. Effect of
Adding Maize Grain or Molasses (at 1 per cent of Liveweight) to a
Basal Diet of Derinded Sugar-cane and Protein Supplement, in the
Three Trials Using Holstein Steers

Control

Improvement over control (%)

Maize

Molasses

Gain in liveweight (kg/day)

Trial 1

.99

27

9

Trial 2

.95

24

13

Trial 3

1.02

32

3

Feed conversion *

Trial 1

9 1

8

- 16

Trial 2

10.1

11

0

Trial 3

9.9

15

- 15

Source: James (9).

* Feed dry matter
consumed/gain in liveweight.

Further evidence for
effects of by-pass starch (maize) on feed conversion rate in
sugarcane diets has been obtained. Leng showed that infusing
glucose directly into the duodenum of lambs receiving a
sugar/straw diet led to better feed conversion, provided that
by-pass protein was also given (12).

In summary, therefore,
we can conclude that, in order to maximize animal productivity on
tropical by-product and waste feeds, small amounts of preformed
by-pass nutrients, specifically protein, must also be provided,
and attention must be given to the roughage characteristics of
the feed.

It is important to
stress that these nutrient/dietary characteristics act rather
like catalysts, in that they create their effect by enhancing the
rate and efficiency of microbial activity within the rumen. A
theoretical example of this is given in Table 3.

TABLE 3. Effect of
By-pass Protein on Efficiency of Synthesis and Total Daily
Production of Rumen Microbial Protein on a Sugar-Cane-Based Diet

Without by-pass protein

With 150 g/day by-pass protein

Turnover rate, times/day

1.5

2.5

Intake of fermentable

CHO, kg/day

1.95

2.63

Microbial synthesis rate,

g N/kg CHO fermented

20

35

Microbial protein, g/day

243

575

It can be seen in this
example that giving 150 g/day of by-pass protein fed to an
increased production of 332 g/day of microbial protein arriving
at the duodenum.

Having identified the
nutritional constraints associated with the use of tropical
byproducts, it becomes possible to plan efficient systems for
converting these into animal products such as beef and milk. This
task also has been facilitated considerably by the recent
findings that both by-pass protein and desired roughage
characteristics can be provided by such protein-rich tropical
forages as the legume shrub Leucaena leucocephala, the leaves of
the banana plant, and the aerial forage part of the sweet potato
(2, 13, 14). Cassava forage is also effective with one specific
tropical by-product, namely molasses (15).

The likely input-output
relationships involved in a rural bioconversion unit using cattle
can be appreciated from the data in Table 4. It is assumed that a
single family unit has 1.5 ha of land and that it will derive its
disposable income from the sale of bananas, milk or cheese, and
beef. The cattle population comprises five adult cows, four
calves, and four steers/heifers. The cattle are confined
throughout the year in an open-side building fitted with a
partially slatted floor so that the faeces and urine fall
directly into a channel below the floor, which, in turn, connects
with the inlet of a biogas digester. After partial digestion, the
final effluent is pumped onto the crops as a fertilizer (see
Figure 4). The same equipment is used to apply irrigation water.
The biogas produced from the unit (about 5.7 m³/day) is
calculated to be sufficient to provide for cooking in the house
and to drive the irrigation/effluent pump and forage chopper.

The cattle are fed a
mixture of sugar-cane (high biomass per unit area), and banana
forage - i.e., the residue after harvesting the fruit, as the
energy source, and sweet potato forage for by-pass protein. No
roots are harvested and the plant is managed as a perennial. Some
additional nonprotein nitrogen such as urea, and phosphorus-rich
minerals and salt are purchased; these are the only imports into
the unit. Both the banana forage and sweet potato forage provide
desirable roughage characteristics.

The advantages of this
particular system are that it:

a. provides a good
level of disposable income for the family;

b. is likely to be
in energy balance (except for the energy cost of the urea and
minerals);

c. avoids erosion by
using perennial forages and by recycling organic matter;

d. relies to a
minimum on imported fertilizer, yet represents a high level
of plant nutrient application because of recycling;

e. reduces
environmental contamination; and

f. uses a minimum of
land area (1.5 ha), yet produces both a cash crop and animal
products.

It was generally agreed
that the presence of substantial amounts of starch in the ration
of ruminants reduces the digestibility of cellulose in the diet,
even when straw has been treated with alkali. While this could be
significant in the feeds given to cattle in the industrialized,
temperate zone countries, animal diets in India are so low in
starch and sugars as to make this warning unnecessary.

The question was raised
about the mechanism that controls voluntary intake of feed by
ruminants. This is not really known, but it is thought to be
controlled by hormonal action.

Concerning the advantage
of by-pass protein, formaldehyde treatment of fish meal and fish
silage have been used for animal feeds. Fish mea) prepared in
this manner has been beneficial for ruminants, but fish silage
has a better quality if left untreated. As untreated fish silage
may have undesirable effects on ruminants, it should be fed only
to nonruminants.

Another beneficial
by-pass protein for ruminant feed is sewage sludge.

Reports from South
Africa indicate that waste water containing algae added to diets
high in sugar-cane residues has improved ruminant growth.